What limits the signal power that a tube can produce? The tube type sets a limit. The maximum plate dissipation of a tube type some what sets an upper limit on the amount of signal power a power tube can pass. For example the max plate dissipation of a 6V6 tube is 12-14 W and the max clean power out for a pair of 6V6’s could get close but will probably never reach 24 or 28 Watts of clean power. The bias also plays a role. A cold biased tube will probably put out less power than a hot biased tube.* I’ve recently done some power output testing with a scope and a signal generator and what stood out to me is that you can view the signal at the speaker and you can see that there is a peak voltage that the amp will not go beyond. The wave flattens out at this invisible line, the RMS voltage can continue to rise as the wave becomes square, but the peak voltage will not rise any farther beyond this one level. The RMS voltage of a square wave is equal to the peak, so the RMS can rise up to the peak but not go above there. Thank you Elpico for showing us how this works. What is it that sets this limit, the limit of the power of the amp? Apparently the place this clipping occurs could be the power tube or could be a pre amp tube. In a simple amp like a champ, the clipping would either have to be the power tube or the second pre amp tube stage. The first stage would never clip because it is never hit with a large signal unless a boost occurs before the 1st stage. What actually sets the limit? Two possibilities out of many: The RMS voltage on the grid cannot exceed the bias voltage on the grid (or between the grid and the cathode) The peak voltage cannot exceed the bias voltage on the grid (or between the grid and cathode) What do you think? I am going to do some testing on a champ power tube, and I know the testing will have one flaw. I can’t be sure if the clipping is in the power tube or a pre amp tube. * I am not 100% sure that this is true.